Characterization of receptors for bombesin/gastrin-releasing peptide in human and murine cells

The role of lipoproteins in the transport and uptake of cyclosporine and dihydro-tacrolimus into HepG2 and JURKAT cell lines

OBJECTIVES

We wish to examine the role of lipoproteins in the transport and cellular uptake of cyclosporine (CsA) and tacrolimus.

DESIGN AND METHODS

The distribution of tritiated CsA and tacrolimus among lipoproteins was determined in normo- and hypertriglyceridemic sera. The uptake of these two drugs into HepG2 and JURKAT cell lines was assessed in the presence of various concentrations of low density lipoproteins. (LDL).

RESULTS

Our data showed that about 60% of these drugs were transported by high density lipoprotein in normolipidemic sera, while about 50-60% were carried by very low density lipoprotein in hypertriglyceridemic sera. Almost 80% of CsA and 70% of tacrolimus entered HepG2 and JURKAT cells within the first hour of incubation in lipoprotein free media. However, the uptake was decreased (CsA by 60% and tacrolimus by 40%) in the presence of LDL.

CONCLUSIONS

Lipoproteins play a major role in the transport of CsA and tacrolimus, but not in their cellular uptake.

Lysis of small cell carcinoma of the lung (SCCL) cells by cytokine-activated monocytes and natural killer cells in the presence of bispecific immunoconjugates containing a gastrin-releasing peptide (GRP) analog or a GRP antagonist

Lung cancer remains the leading cause of cancer deaths in the United States. We have developed a new immunotherapeutic approach to the treatment of small cell carcinoma of the lung (SCCL) by targeting the gastrin-releasing peptide receptor (GRP-R) expressed on the surface of these cells. Bispecific immunoconjugates were constructed by chemical fusion of a GRP analog or a GRP antagonist with monoclonal antibodies directed to the cytotoxic trigger molecules Fc gamma RI and Fc gamma RIII on various immune effector cells. We demonstrated that these bispecific immunoconjugates bound to target SCCL cells in a dose-dependent manner. In the presence of these immunoconjugates, more than 80% of SCCL cells were lysed by cytokine-activated monocytes and natural killer (NK) cells measured by a 51Cr-release assay. These data indicate that bifunctional antibodies targeting GRP may have clinical use.

Potent bombesin antagonists with C-terminal Leu-psi(CH2-N)-Tac-NH2 or its derivatives

Various pseudononapeptide bombesin (BN)-(6-14) antagonists with a reduced peptide bond (CH2-NH) between positions 13 and 14 can suppress the mitogenic activity of BN or gastrin-releasing peptide in 3T3 fibroblast cells and small cell lung carcinoma. In the search for more potent BN antagonists, 10 modified nonapeptide BN antagonists containing N-terminal D-Phe, D-Cpa, and D- or L-Tpi and C-terminal Leu-psi(CH2-N)-Tac-NH2, Leu-psi(CH2-N)-MeTac-NH2, or Leu-psi(CH2-N)-Me2Tac-NH2 have been synthesized by incubating [13 psi 14,CH2-NH,Cys14]BN-(6-14) or [13 psi 14-CH2-NH,Pen14]BN-(6-14) with formaldehyde or acetaldehyde (Cpa = 4-chlorophenylalanine, Tac = thiazolidine-4-carboxylic acid, Tpi = 2,3,4,9-tetrahydro-1H- pyrido[3,4-b]indol-3-carboxylic acid, and Pen = penicillamine). The biological activities of these compounds were then evaluated. [D-Phe6,13 psi 14,CH2-N,Tac14]BN-(6-14) (RC-3950-II) and [D-Phe6,13 psi 14,CH2-N,Me2Tac14]BN-(6-14) (RC-3985-II) exhibited greater potency in inhibition of 125I-labeled [Tyr4]BN binding to Swiss 3T3 cells than their parent compounds [D-Phe6,13 psi 14,CH2-NH,Cys14]BN-(6-14) (RC-3950-I) and [D-Phe6,13 psi 14,CH2-NH,Pen14]BN-(6-14) (RC-3985-I). The order of binding affinities of these compounds was as follows: [13 psi 14,CH2-N,Tac14]BN-(6-14) > [13 psi 14,CH2-N,Me2Tac14]BN-(6-14) > [13 psi 14,CH2-N,MeTac14]BN-(6-14). In most cases, the analogs with C-terminal Leu-psi(CH2-N)-Tac-NH2 were also more potent growth inhibitors of 3T3 cells than compounds containing C-terminal Leu-psi(CH2-N)-Me2Tac-NH2 or Leu-psi(CH2-N)-MeTac-NH2. The best BN antagonists of this series, RC-3950-II and [D-Cpa6,13 psi 14,CH2-N,Tac14]BN- (6-14) (RC-3925-II), inhibited gastrin-releasing peptide-stimulated growth of Swiss 3T3 cells with IC50 values of 1 nM and 6 nM, respectively. Since antagonists of this class inhibit growth of various tumors in animal cancer models, some of them may have clinical applications.

Novel soluble, high-affinity gastrin-releasing peptide binding proteins in Swiss 3T3 fibroblasts

Swiss 3T3 cells contained substantial amounts of soluble and specific [125I]GRP binders. Like the membrane-associated GRP receptor, they were of high affinity, saturable, bound to GRP(14-27) affinity gels, and exhibited specificity for GRP(14-27) binding. They differed in that acid or freezing destroyed specific binding, specific binding exhibited different time and temperature effects, no detergent was required for their solubilization, ammonium sulfate fractionation yielded different profiles, the M(rs) were lower, GRP(1-16) also blocked binding, and a polyclonal anti-GRP receptor antiserum did not bind on Western blots. The isolated, soluble GRP binding protein(s) rapidly degraded [125I]GRP. These soluble GRP binding proteins may play a role in the regulation of the mitogenic effects of GRP on these cells.

Molecular cloning and characterization of receptors for the mammalian bombesin-like peptides

The bombesin-like peptides comprise a large family of peptides common to both amphibians and mammals that function as growth factors, neurotransmitters, and paracrine hormones. GRP, the mammalian homolog of bombesin and its receptor, as well as NMB, the mammalian homolog of ranatensin, are expressed in human neoplasms and, in particular, in small cell lung carcinomas (SCLC). To better characterize the physiological roles of bombesin-like peptides, our laboratory has cloned the receptors for GRP in murines, rats, and humans. The 3T3 GRP receptor was isolated and characterized using the two-electrode-voltage-clamp analysis and acquorin-emission methods in xenopus oocytes expression system. The rat and human GRP and NMB receptors were cloned by hybridization at low stringency, using the mouse cDNA receptor probe. Sequence analysis of the receptors showed 384 and 390 amino acids for GRP and NMB receptors, respectively. The homology between the two receptors is 60% and between species in the same receptor, 90%. The receptors belong to the 7-membrane spanning domains superfamily. The specific GRP-R antagonist blocked the response to bombesin in oocytes injected with GRP-R, but failed to do so in oocytes injected with NMB-R. The two receptors differ in their distribution of tissue expression. RNA blot and RNase protection analysis showed the same size of mRNA without alteration in the receptors. RT + PCR analysis performed on genomic DNA revealed similarity between normal and cell DNAs, suggesting no major gene deletion or rearrangement. Southern blot analysis indicated the absence of gene amplification. Sequence analysis of the exonic segments of the receptor genes displayed identical amino acids to the respective cDNAs. None of the genes had classic TATAA box. Somatic cell hybrids localized the GRP-R on the X-chromosome and the NMB-R on chromosome 6. The same sequence of normal genes and cDNAs of GRP and NMB receptors, together with the gene characterization, demonstrated that SCLC cell lines do not require a structural change in receptor protein or genomic rearrangement.

Growth factor and peptide receptors in small cell lung cancer

In the past decade, over 1000 continuous human cell lines have been established from lung cancer biopsy specimens. Numerous growth factors and receptors have been identified in the small cell lung cancer (SCLC) cell lines. SCLC is a neuroendocrine tumor which contains numerous peptides, including bombesin/gastrin releasing peptide (BN/GRP), and receptors. High levels of GRP mRNA and immunoreactivity are present in SCLC cells. The secretion rate of GRP from SCLC cells is increased by vasoactive intestinal peptide (VIP), which elevates the intracellular cAMP. GRP binds to cell surface receptors, elevates cytosolic calcium and stimulates the growth of SCLC cells. Additional SCLC growth factors include insulin-like growth factor I (IGF-I) and transferrin. IGF-I mRNA and protein is present in SCLC. IGF-I binds with high affinity to SCLC cells and stimulates tyrosine kinase activity and growth. Transferrin is also present in SCLC cells. Transferrin binds with high affinity to SCLC cells and stimulates iron transport and growth. Synthetic peptide antagonists and monoclonal antibodies have been identified which disrupt autocrine growth pathways and inhibit SCLC growth.

Pseudononapeptide bombesin antagonists containing C-terminal Trp or Tpi

Seven new antagonists of bombesin (Bn)/gastrin-releasing peptide (GRP) containing C-terminal Trp or Tpi (2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indol-3-carboxylic acid) in a reduced peptide bond were synthesized by solid phase methods and evaluated biologically. The reduced bond in four [Leu13 psi(CH2NH)Trp14]Bn(6-14) analogs was formed by reductive alkylation at the dipeptide stage. In the case of three [Leu13 psi(CH2N)Tpi14]Bn(6-14) analogs, the Trp dipeptide with reduced bond was reacted with formaldehyde to form the corresponding Tpi derivative. These Tpi-containing analogs have a new reduced bond which is structurally more constrained. Leu13 psi(CH2N)Tpi14 analogs inhibit [125I][Tyr4]bombesin binding to Swiss 3T3 cells with IC50 values of 2-4 nM, compared to 5-10 nM for Leu13 psi(CH2NH)Trp14 analogs. Leu13 psi(CH2N)Tpi14 analogs are also more potent than Leu13 psi(CH2NH)Trp14 analogs in growth inhibition studies using Swiss 3T3 cells. The two best bombesin antagonists of this series, [D-Trp6,Leu13 psi(CH2N)Tpi14]Bn(6-14) (RC-3415) and [Tpi6,Leu13 psi(CH2N)Tpi14]Bn(6-14) (RC-3440), inhibited GRP-stimulated growth of Swiss 3T3 cells with IC50 values less than 1 nM. RC-3440 was also active in vivo, suppressing GRP(14-27)-stimulated serum gastrin secretion in rats. Bombesin/GRP antagonists, such as RC-3440, containing the new reduced bond (CH2N) reported herein are very potent.

Inhibition of growth of PC-82 human prostate cancer line xenografts in nude mice by bombesin antagonist RC-3095 or combination of agonist [D-Trp6]-luteinizing hormone-releasing hormone and somatostatin analog RC-160

The effects of treatment with a bombesin receptor antagonist [D-Tpi6, Leu13 psi (CH2NH) Leu14]BN(6-14)(RC-3095) and the combination of an agonist of luteinizing hormone-releasing hormone [D-Trp6]-LH-RH and somatostatin analog D-Phe-Cys-Tyr-D-Trp-Lys-Val- Cys-Trp-NH2 (RC-160) were studied in nude mice bearing xenografts of the hormone-dependent human prostate tumor PC-82. During the 5 weeks of treatment, tumor growth was decreased in all treated groups compared with controls. Bombesin antagonist RC-3095 and the combination of [D-Trp6]-LH-RH and RC-160 caused a greater inhibition of tumor growth than [D-Trp6]-LH-RH or RC-160 alone as based on measurement of tumor volume and percentage change in tumor volume. The largest decrease in tumor weight was also seen in the groups treated with the bombesin antagonist and with the combination of RC-160 and [D-Trp6]-LH-RH. Serum prostatic-specific antigen levels were greatly decreased, and insulin-like growth factor I (IGF-I) as well as growth hormone levels were reduced in all treated groups. Specific binding sites for [D-Trp6]-LH-RH, epidermal growth factor (EGF), IGF-I, and somatostatin (SS-14) were found in the tumor membranes. Receptors for EGF were significantly down-regulated by treatment with the bombesin antagonist or RC-160. Combination of LH-RH agonists with somatostatin analog RC-160 might be considered for improvement of hormonal therapy for prostate cancer. The finding that bombesin antagonist RC-3095 inhibits the growth of PC-82 prostate cancer suggests the merit of further studies to evaluate the possible usefulness of antagonists of bombesin in the management of prostatic carcinoma.

Biological effects and receptor binding affinities of new pseudononapeptide bombesin/GRP receptor antagonists with N-terminal D-Trp or D-Tpi

In an attempt to produce more powerful (effective) bombesin/GRP receptor antagonists, the D forms of Trp or Trp analog (Tpi) were introduced at position 6 in two pseudononapeptides, Leu13 psi (CH2NH)Leu14-bombesin(6-14) and Leu13 psi(CH2NH)Phe14-bombesin (6-14). These antagonists were tested for their ability to inhibit basal and gastrin releasing peptide (GRP) (14-27)-induced amylase release from rat pancreatic acini in a superfusion assay. They were also assessed for the inhibition of 125I-Tyr4-bombesin binding to Swiss 3T3 and small cell lung carcinoma cell line H-345 and the mitogenic response of Swiss 3T3 cells induced by GRP(14-27). The peptides, when given alone, did not stimulate amylase secretion, but were able to inhibit gastrin releasing peptide (14-27)-induced amylase release. All of the antagonists showed strong binding affinities for Swiss 3T3 and H-345 cells and suppressed the GRP(14-27)-induced increase of [3H]thymidine incorporation into DNA of Swiss 3T3 cells at nanomolar concentrations. Antagonist D-Tpi6,Leu13 psi (CH2NH)Leu14-bombesin (6-14)(RC-3095) was slightly more potent in these assays than D-Trp6,Leu13 psi (CH2NH)Leu14-bombesin (6-14)(RC-3125). Nevertheless, D-Trp6,Leu13 psi (CH2NH)Phe14-bombesin (6-14) showed the highest binding affinity for Swiss 3T3 and H345 cells and it was the most potent inhibitor of GRP(14-27)-induced amylase secretion. This antagonist RC-3420 was particularly effective in inhibiting the growth of Swiss 3T3 cells, exhibiting an IC50 value less than 1 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotinylation of a bombesin/gastrin-releasing peptide analogue for use as a receptor probe

The development of a biotinylated bombesin/gastrin-releasing peptide (GRP) for use as a receptor probe is reported. The lysine13 of a GRP-27 was substituted by arginine and lysine was added to the amino terminus. Biotinylation of the N-terminal lysine was performed. The biotinylated peptide was purified by HPLC and characterized by mass spectral analysis. Binding studies with murine Swiss 3T3 fibroblasts, cells known to express bombesin/GRP receptors, yielded a dissociation curve for the biotinylated GRP-27 analogue (biotin-Lysyl[Asp12,Arg13]GRP-27) which was nearly identical to that of native GRP. Using studies of gastrin release from isolated canine G cells, equipotent functional activity of the biotinylated probe and unmodified GRP was demonstrated. Measurements of retained 125I-avidin confirmed that the biotin/avidin interaction could occur once the biotin-peptide complex was bound. Applicability of the probe was demonstrated with fluorescent microscopy using avidin-FITC on Swiss 3T3 fibroblasts. In conclusion, a novel biotinylated bombesin/GRP analogue has been developed which retains the functional characteristics of the native peptide and is a useful probe for receptor studies.

Characterization of the bombesin receptor on mouse pancreatic acini by chemical cross-linking

Bombesin (BN), gastrin-releasing peptide (GRP) and GRP(18-27) (neuromedin C) were equipotent and 30-fold more potent than neuromedin B (NMB) in inhibiting binding of 125I-GRP to and in stimulating amylase release from mouse pancreatic acini. In the present study we used 125I-GRP and chemical cross-linking techniques to characterize the mouse pancreatic BN receptor. After binding of 125I-GRP to membranes, and incubation with various chemical cross-linking agents, cross-linked radioactivity was analyzed by SDS-PAG electrophoresis and autoradiography. With each of 4 different chemical cross-linking agents, there was a single broad polypeptide band of Mr 80,000. Cross-linking did not occur in the absence of the cross-linking agent. Cross-linking was inhibited only by peptides that interact with the BN receptor such as GRP, NMB, GRP(18-27) or BN. Dose-inhibition curves for the ability of BN or NMB to inhibit binding of 125I-GRP to membranes or cross-linking to the 80,000 polypeptide demonstrated for both that BN was 15-fold more potent than NMB. The apparent molecular weight of the cross-linked polypeptide was unchanged by adding dithiothreitol. N-Glycanase treatment reduced the molecular weight of the cross-linked peptide to 40,000. The present results indicate that the BN receptor on mouse pancreatic acinar cell membranes resembles that recently described on various tumor cells in being a single glycoprotein with a molecular weight of 76,000. Because dithiothreitol had no effect, this glycoprotein is not a subunit of a larger disulfide-linked structure.

Bradykinin receptors undergo ligand-induced desensitization

Bradykinin binds to specific cell surface receptors on Rat13 fibroblasts with a high affinity (2.1 nM). Prolonged exposure of cells to the ligand causes a concentration-dependent decline in surface levels of the 2.1 nM receptor from 40,000 receptors per cell to undetectable levels with a t1/2 of approximately 2 h. The decline occurs in parallel with the appearance of an equal number of lower affinity binding sites (40 nM), suggesting that ligand exposure causes desensitization by an alteration in receptor affinity. The affinity change is characterized by a faster rate of ligand dissociation while the rate of association remains unaltered. The observed desensitization is dependent on the presence of active cellular metabolism since (i) it does not occur in whole cells maintained at 4 degrees C and (ii) membranes prepared from Rat13 cells retain their high-affinity sites at 37 degrees C despite extensive ligand exposure.